SINGAPORE --Singapore authorities say a 27-year-old man has SARS, the world's first case since a global outbreak was declared over in July.

Singaporean officials confirmed the laboratory technician had SARS after results from a second test were released Tuesday, and have ordered 25 people who have been in contact with the man to stay at home.

So far, none of them is ill, and it is unclear how the man -- an unnamed post-doctoral student working on the West Nile virus -- caught the flu-like illness.

"It appears to be a single isolated case," the Ministry of Health said in a statement.

The World Health Organization (WHO) has said the results are "perplexing," with the Singapore case not fitting its rigid clinical definition of severe acute respiratory syndrome, or SARS.

The man "doesn't have all the symptoms that we would expect to see in a SARS case as we understand it," WHO spokesman Iain Simpson told CNN by phone from Geneva.

According to WHO criteria, the man would have to have inflamed lungs, which he did not have, although he had been suffering from fever and rash.

"However, it is clear that the virus the lab has identified is the virus that causes SARS," Simpson said.

"So, it may be that SARS is causing a slightly different or a slightly less severe form of illness at this point."

Originating in the southern Chinese province of Guangdong, SARS killed 840 people, and sickened 8,500 in 30 countries, before the World Health Organization declared it was over in July.

The man had checked into the accident and emergency department at Singapore General before being taken late Monday to Tan Tock Seng Hospital, Singapore's dedicated SARS facility.

Three wards have been closed at Singapore General Hospital, the city's largest public hospital, with emergency isolation procedures in place.

The Singaporean victim has not traveled recently, nor has he had any known contact with SARS patients, according to acting Minister of Health Khaw Boon Wan.

Khaw said he thought the man posed a "low public health risk" because he was isolated quickly.

The SARS epidemic was declared under control by the WHO on July 5.

While SARS virus samples were handled at the laboratory he worked in, the man was not one of the four people involved in SARS research.

Singapore will send the samples of the man's tissue to the U.S. Centers for Disease Control and Prevention for further tests.

Singapore has some of the strictest anti-SARS measures in place, and last reported a SARS case in early May.

But scientists say because they know so little about the disease, and because there is no vaccine for it, they are worried about it returning.

The flu-like illness is thought to have jumped from animals to humans in China late last year.

The epidemic crippled travel and tourism to the region, especially in East Asia, with tourists and business travelers avoiding the area.

In Singapore -- one of the worst affected areas after China, Taiwan and Hong Kong -- the virus killed 33 people and infected 238.